This invention relates to the field of processing readback signals representing data recorded in magnetic recording devices and, in particular, to binary data signal conditioning and detection. Improved performance and capability of magnetic recording devices, in particular, increased capacity by increasing density of recorded flux transitions representing data, have been achieved largely through advances in head and media technology. As further advances in head technology, head-to-media spacing, media surface smoothness and the like have become more difficult to achieve, signal processing techniques now provide opportunities for further improvement.
As density of recorded data is increased, pulses in the readback signal derived from the recorded transitions become more crowded causing intersymbol interference between adjacent pulses. Intersymbol interference caused by pulse crowding causes peak shift of the pulses and reduces pulse amplitude, both of which degrade data retrieval.
Higher density of recorded data produces readback signals of correspondingly higher frequency. Thus, the frequency response of read channel electronic circuitry must be adequate to avoid degradation of the signal and difficulties in data detection.
Signal processing of readback signals has in the past included pulse shaping, filtering and amplifying in various combinations to mitigate intersymbol interference and related effects of higher density data recording. For example, U.S. Pat. No. 3,775,759 discloses tapped delay line equalization in which a readback signal is filtered and incidentally, slimmed. When readback pulses are slimmed, their rising and falling edges are moved closer together, and are less apt to interfere with adjacent pulses.
U.S. Pat. No. 4,264,935 discloses slimming of pulses using a plurality of summed delays as well as a differentiator to produce a slimmer pulse representing data. The use of delay for slimming is also described in U.S. Pat. No. 3,828,362 and in "The Complete Handbook of Magnetic Recording" by Finn Jorgensen at page 276.
In U.S. Pat. No. 4,202,017, a pulse equalization technique is described in which negative-polarity read pulses are symmetrically positioned around, and algebraically added to, positive-polarity read pulses to produce a single composite read pulse which is substantially narrower than the initial positive polarity read pulse. Pulse slimming, achieved by using the first and second derivatives of the smoothed readback pulse is described in "An Improved Pulse-Slimming Method for Magnetic Recording" by Richard C. Schneider, published in IEEE Transactions on Magnetics, Vol. Mag-11, No. 5, September 1975.
Filtering of readback pulses permits higher density data recording. See, for example, U.S. Pat. No. 4,244,008 which teaches filtering as part of a readback compensation circuit. Finally, U.S. Pat. No. 4,327,383 describes a combination of several signal processing functions in the readback path including an amplifier, an equalizing filter, a low pass filter and a decoder. Readback signal pulse slimming techniques inherently cause decrease of signal-to-noise (S/N) ratio. Of course, S/N is also affected by other system components, including encoding and detection devices.